The present invention relates generally to glass compositions having inert fillers incorporated therein and suitable for the direct passivation of semiconductor surfaces and more particularly, to glass compositions containing cordierite and a thermal expansion compatible with the thermal expansion of silicon and whereby said glass compositions can be applied as coatings for semiconductor devices, and particularly silicon semiconductor devices.
A number of glasses are useful for passivating semiconductor surfaces, i.e., protect semiconductors from any effects of their surroundings. Glasses have been preferred for this purpose in the past. Glasses suitable for providing a passivating and protective coating for the underlying semiconductor unit should have a high electric insulation. In addition, the coefficient of thermal expansion of the glasses should match or complement the coefficient of thermal expansion of the semiconductor material, which is being passivated with said glass so that any applied glass layers coatings will not split off or crack during the cooling process or when exposed to variations in temperature.
The glasses used in the prior art for protecting and passivating silicon semiconductor materials do not ususally have a coefficient of thermal expansion matching or complementing that of silicon because silicon has a relatively small coefficient thermal expansion (about 33 .times. 10.sup.- /.degree. C.). Although some glasses are known which are similar to silicon in their heat expansion, these glasses are still not satisfactory because they have such high fusing temperatures, i.e., markedly above 1000.degree. C., that they cannot be applied on the semiconductor without causing permanent damage thereto. Thus, only those glasses capable of being used for passivating Si-semiconductors are those which have fusing temperatures below 1000.degree. C. The fusing temeprature of these glasses is the temperature at which the glass powder has softened and liquefied to such a degree that it creates a dense layer, which is free of pores, on the semiconductor surface. However, these glasses as a rule have thermal expansion coefficients which are clearly greater than those for silicon with the values increasing as the fusing temperature decreases.
For example, the first group of glasses, which is frequently used for Si-thyristors and which consists basically of about 40 to 60% by weight PbO, about 5 to 15% by weight Al.sub.2 O.sub.3 and about 30 to 50% by weight Si0.sub.2, has a thermal expansion value between 45 and 50 .times. 10.sup.-7 /.degree. C. at fusing temperatures of about 900.degree. C. The second and third important groups of glasses, i.e., zinc-silicoborate glasses containing about 60% by weight of ZnO, 20 to 30% by weight of B.sub.2 O.sub.3 and 5 to 15% by weight of SiO.sub.2, and lead-borosilicate glasses with about 40 to 50% by weight of PbO, about 40% by weight of SiO.sub.2 and about 10% by weight of B.sub.2 O.sub.3, have thermal expansion values of about 45 .times. 10.sup.-7 /.degree. C. at fusing temperatures of about 650 .degree. to 720.degree. C. Such glasses are frequently utilized for rectifiers, diodes and transistors. The fourth group of glasses, i.e., again lead-borosilicate glasses, but with a higher PbO content of about 70% by weight, has thermal expansion values of more than 60 .times. 10.sup.-7 /.degree. C. at fusing temperatures between 500.degree. and 600.degree. C. The latter group of glasses is the preferred material for Si-semiconductor elements, because they are able to withstand particularly low temperature stresses as a result of their thermal sensitivity or because metal contacts have already been applied.
Due to the deviations in thermal expansion which exist in all cases, all of the glasses used in the prior art are compatible with silicon only in the form of thin layers, with the permissible layer thickness becoming progressively reduced as the difference in expansion coefficients becomes greater. For example, when using glasses of the first group mentioned above, which has the highest fusing temperatures, it is possible to use layers of as much as 50 .mu.m without having cracks or chipping develop during cooling or when exposed to temperature changes, whereas the glasses of the fourth group may be used only in layers of about 1 .mu.m in thickness.
The restriction to the use of such thin layers creates considerable problems for production engineering. Therefore, expensive processes, e.g., sedimentation, centrifuging or electrophoresis processes have to be used to precipitate, from a suspension, a uniform layer of glass powder onto the semiconductor element, which subsequently can be melted thereon to form a smooth layer which is completely free of pores. It becomes increasingly more difficult to avoid the formation of pores as the layers which are applied become thinner and thinner, and it is possible to achieve pore-free layers only if extremely fine grain sizes of the glass powder are used.
It would therefore be highly advantageous to be able to apply thicker glass layers in many cases, especially since the layers would be less susceptible to mechanical damage. Also, with regard to the electrical aspects, the use of thicker layers would provide better protection for the semiconductor unit because the electrical fields, which escape on the semiconductor surface, would be able to decay within the layer of glass and therefore would not escape to the outside, or become greatly reduced in intensity.